This invention relates to a filter sheet and process for purifying a photoresist composition employing said filter sheet and, more particularly, to the removal of ionic contaminants from photoresist compositions utilizing said filter sheet, said filter sheet comprising a self-supporting fibrous matrix having immobilized therein particulate filter aid and particulate ion exchange resin.
Photoresist compositions are used extensively in integrated circuit manufacture. Such compositions typically comprise a light-sensitive component and a polymer binder dissolved in a polar organic solvent. Typical photoresist compositions are disclosed in U.S. Pat. Nos. 5,178,986, 5,212,046, 5,216,111 and 5,238,776, each incorporated herein by reference for disclosure of photoresist compositions, processing and use. Impurity levels in photoresist compositions are becoming an increasingly important concern. Impurity contamination, especially by metals, of photoresists may cause deterioration of the semiconductor devices made with said photoresists, thus shortening these devices"" lives. Impurity levels in photoresist compositions have been and are currently controlled by (1) choosing materials for photoresist compositions which meet strict impurity level specifications and (2) carefully controlling the photoresist formulation and processing parameters to avoid the introduction of impurities into the photoresist composition. As photoresist applications become more advanced, tighter impurity specifications must be made.
Efforts to remove dissolved ionic contaminants from organic liquids, such as photoresist compositions, are known in the art. For example, U.S. Pat. No. 5,045,210 discloses an ion exchange media containing a modified polysaccharide material and a modified silica or modified particulate polymeric material, said modified materials including a polysaccharide and silica or particulate polymer material covalently bonded to a synthetic polymer having chemical groups capable of coupling to the polysaccharide, silica and/or particulate polymer.
U.S. Pat. Nos. 5,284,930 and 5,288,850 disclose processes for removing metals from photoresist vinyl phenol polymer solutions which entail passing the solutions through an ion-exchange column comprising a strongly acidic cation exchange resin.
U.S. Pat. No. 5,346,624 describes a method for removing ionic and suspended particulate impurities from a liquid by passing the liquid through a bed comprising a mixture of powdered ion exchange resin particles, ion exchange fibers and relatively inert fibers.
U.S. Pat. No. 5,350,714 describes a process whereby organic solutions are treated to remove contaminants immediately prior to use of the solution (xe2x80x9cpoint-of-use purificationxe2x80x9d). The process involves the use of contaminant removal means at the point of end use of the solution, the contaminant removal means containing a bed comprising ion exchange resin, activated carbon or mixtures thereof.
U.S. Pat. No. 5,376,278 discloses a method of removing charged particles from a liquid stream by passing the liquid stream through a filter sheet which is conditioned by pre-coating an ion exchange material on the surface of the filter sheet.
U.S. Pat. No. 5,378,802 discloses a method of removing ionic impurities from a photoresist which involves dissolving the photoresist in a solvent, contacting the resulting photoresist composition resist component with a fibrous ion exchange resin and separating the ion exchange resin bearing the metal impurities from the photoresist composition.
U.S. Pat. No. 5,500,127 describes a process for purifying an acid catalyzed photoresist which comprises passing the photoresist through a column containing a weak acid cation exchange resin and a strong base anion exchange resin.
U.S. Pat. No. 5,521,052 discloses a process for producing novolak resins containing low levels of metal ions. The process entails passing novolak resin through an anion exchange resin which has been treated with deionized water and mineral acid followed by passing the solution through a cation exchange resin which has been treated with deionized water and mineral acid.
U.S. Pat. No. 5,525,315 discloses a process for removing dissolved heavy metal ions from an organic solution which involves passing the organic solution through a bed containing cation exchange resin having chelating groups on the resin or slurrying the organic solution with a solution containing the chelating exchange resin.
Filter media previously employed in the filtration of photoresists are known which comprise a self-supporting matrix of celluose fibers having immobilized therein particulate filter aid and binder resin. Such filter media are sold under the tradename Zeta Plus(copyright) EC Series (CUNO, Inc., Meriden, Conn., U.S.A.) and do not contain ion exchange resin embedded therein.
In accordance with the present invention, a filter sheet is provided which comprises a self-supporting fibrous matrix having immobilized therein particulate filter aid and particulate ion exchange resin, wherein said particulate filter aid and particulate ion exchange resin are distributed substantially uniformly throughout a cross-section of said matrix. It has been surprisingly discovered that the filter sheet of this invention exhibits a significantly higher ion exchange capacity compared to the ion exchange capacity of the ion exchange resin alone, notwithstanding the fact that the other major constituents of the subject filter sheet, namely, the fibers utilized to form the fibrous matrix and the particulate filter aid, possess little or no ion exchange capacity themselves. Further in accordance with the present invention, a process for removing ionic impurities, e.g., metal cations, from an organic liquid, e.g., a photoresist solution, is provided which comprises passing the ionic impurity-containing organic liquid through the filter sheet to provide a purified photoresist composition. The process of this invention results in the reduction of ionic impurities down to low parts per billion levels in photoresist compositions.